Which one of the following is the basic vertical reference in all forms of heading and attitude devices?

Line perpendicular to actual horizon

Line perpendicular to artificial horizon

Earth‚Äö√Ñ√∂‚àö√ë‚àö¬•s gravitational field

Earth‚Äö√Ñ√∂‚àö√ë‚àö¬•s magnetic field

Which one of the following is the basic vertical reference in all form

1.	Which one of the following is the basic vertical reference in all forms of heading and attitude devices?
A.	Line perpendicular to artificial horizon
B.	Earth‚Äö√Ñ√∂‚àö√ë‚àö¬•s gravitational field
C.	Line perpendicular to actual horizon
D.	Earth‚Äö√Ñ√∂‚àö√ë‚àö¬•s magnetic field
Answer» C. Line perpendicular to actual horizon